A/C Compressor? on 1997 Ford F-150

MY A/C shuts off on VERY hot days or somtimes just warms up slowly until nothing but hot air blows. Checked at shop... it's fully charged and blows ice cold air on moderate days. I'm thinking I have a seal leak in the compressor.

I've seen this happen when the a/c clutch is failing. What happens is that when the a/c has been on for awhile, the clutch coil heats up and then goes "open circuit", causing the compressor to stop turning. When the problem starts, is the compressor on? If not, check the voltage supply to the clutch and if it is getting voltage, the clutch is bad.